Welded profile wire screens having downwardly sloping or directed screen surfaces formed by laterally spaced transverse wedge-shaped profile wires or rods, are widely used as static screens in classifying and/or deliquefying liquid-solid mixtures fed as slurries to the screen surface for flow by gravity downwardly therealong transversely of or across the screen wires. Such screens depend for classification or size separation on the size of the slots or openings between the screen wires and the velocity of the slurry and separate liquid and undersized particles by a slicing or shearing action exerted by the screen wires on the underside or bottom of the slurry. For removing coarse solids of a size down to about 65 mesh, a sufficient velocity of flow of the slurry is obtained by a gravity feed of the slurry onto the screening surface from an overlying feed box. However, for separating fine fibrous or other non-abrasive solids of a size less than about 100 or 120 mesh with the screen openings ranging in width from about 150 down to about 50 microns, the necessary higher velocity requires the slurry to be pressure fed to the screen surface, as through a plurality of laterally spaced nozzles. Whether gravity or pressure fed, a static profile wire screen may be flat but more usually is a so-called "sieve bend" having a concave screen surface and in either case the slurry is fed or directed onto the screen surface at a tangent or parallel thereto.
Although a pressure fed fine opening profile wire screen is capable of fine separations, a problem is confronted when the solids in the slurry are in part slimes which have a pronounced affinity for and will adhere to almost any surface they contact. In general metallurgical practice, slimes, as distinguished from sands, are considered as any material finer than 200 mesh. However, the slimes encountered as solid suspensions in liquid carriers in solid-liquid mixtures, such as sewage, trade wastes and coal slurry, the last clay slimes derived from clays contained as a nonvolitile contaminant in the coal, usually are about 4 to 5 microns or less in particle size and thus sufficiently fine to enter the 50 micron slots of the finest of pressure fed screens now in general use.
The problem posed by the slime content of a slurry in a fine separation by a pressure fed screen is that the purpose of the pressure feed is to increase the velocity of flow of the slurry along the screen for enhancing the shearing action of the wedge-shaped screen wires and, with the feed tangential or parallel to the screening surface, there is no force for driving the separated liquid and undersized solids through the screen openings except along the leading sides of the screen wires. Consequently, in fine screening as presently practiced on profile wire screens, the removed liquid and suspended solids flow mainly by gravity downwardly along the leading sides of the screen wires and then along the backs of the wires forming the back of the screen. Any contained slimes thus are afforded an opportunity to adhere to the sides and backs of the screen wires and progressively blind or seal off the screen openings or slots from both front and back. It is to a solution of this problem that the present invention is particularly directed.